This invention relates generally to the field of safe handling of chemicals, and more particularly to a method and apparatus for preventing fire and explosions in the production, purification and transportation of unstable hydrides by subdividing the enclosure containing the hydride and providing localized heat storage and heat transfer means to remove heat from an incipient fire or explosion so as to prevent propagation of the aforesaid fire or explosion.
The invention relates to a method for preventing the occurrence of a fire or explosion in an enclosure. The term “enclosure” as used herein describes a space having a boundary that substantially encloses the perimeter of the space such as a storage tank, a cylinder, a duct, a cavity, or a vessel, which may be at any pressure. The enclosure has one or more openings to permit the ingress and egress of materials and said openings may be sealed by devices such as flanges or valves and may be connected in series with other enclosures.
The problem of fire and explosions in enclosed spaces has been a serious issue for many years. It was recognized that these explosions required a source of ignition and also a sufficient amount of the flammable gas and air. Thus the “Lower Explosive Limit” of a particular gas is defined as the minimum amount of the gas mixed with air at room temperature and pressure, which would explode in the presence of a source of ignition and is referred to as “LEL”. The “Upper Explosive Limit” of a particular gas is defined as the maximum amount of the gas mixed with air at room temperature and pressure, which would explode in the presence of a source of ignition and is referred to as “UEL”. Thus it was recognized that it was possible to have a safe mixture of potentially explosive gas with air providing the concentration of the gas was either below the LEL or above the UEL. The LEL and UEL provide useful concepts for room temperature and pressure situations in air and can be modified to account for temperature pressure and the presence of other gases. The addition of a gas to another gas is termed “diluting” and the added gas is the “diluent”. Zabetakis provides in Bureau of Mines Bulletin 627 flammability, data for more than 200 combustible gases and vapors and graphs and empirical rules that can be used for adjusting the LEL and UEL for gases under different conditions from the standard room temperature and dilution with air. It is thus possible to adjust the flammability range. The bulletin also notes that for gases such as hydrazine, which can burn in the absence of an oxidizer, the UEL is 100% but it is possible to add stable diluents that will render the mixture nonflammable. The effect of tube diameter on the decomposition of pure acetylene is shown in FIG. 61 of the Bulletin and shows the inverse log—log relationship of pressure and diameter.
It was also recognized that fire and explosions were related phenomenon with explosions being extremely rapid fires. Thus since it was well known that water and other chemicals would put out fires, the idea of suppressing explosions with water or other chemicals has been studied and can be effective providing the explosion is detected and suppressed before it builds up to destructive power.
There are two types of technology for safe handling of potentially hazardous gases in enclosures, the prevention approach and the suppression approach. In the prevention approach the technology attempts to prevent a fire or explosion from starting by removing the source of ignition or by operating in a non-flammable regime. Removing the source of ignition may mean use of intrinsically safe, low voltage, sensors, protecting against static discharge and external insulation to protect against fire and operating below the auto ignition temperature, the temperature at which a material explodes without a source of ignition. The main advantage of the removal of a source of ignition is that it is relatively low cost; its major disadvantage is that there may be an unexpected source of ignition in which case there is no protection. The unexpected sources of ignition can be a catalytic reaction with the wall of the enclosure or with corrosion products on the wall, such reactions typically form hot spots that slowly t accumulate heat until a fire or explosion can propagate.
One common example of operating in a non-flammable regime is the use of nitrogen “blankets” over flammable liquids stored in atmospheric pressure tanks to prevent fires. This is an example of dilution with nitrogen of a flammable vapor and air mixture. Sufficient nitrogen is provided to raise the LEL of the mixture so that the mixture is not flammable at the operating conditions of the tank. For sealed gas cylinders, certain gases that decompose without an oxidizer, i.e. an upper UEL of 100%, are shipped in a diluted form such that they are non-flammable. Alternatively they may be shipped or used at reduced pressure since reducing the pressure has been shown to reduce the risk of deflagration, as shown in FIG. 61 of Zabetakis for Acetylene. They may also only be shipped in small cylinders since lowering the diameter lowers the risk.
The advantage of operating in the nonflammable regime is that it is a passive system and will prevent fires and explosions within the conditions it is designed for. However the lack of purity is particularly a problem during production and purification of the flammable materials, particularly materials, which are flammable without an oxidizer. The standard purification techniques of distillation and membrane separation can be carried out in the substantial absence of air fairly readily for materials that are stable in the absence of an oxidant because it is possible to operate above the UEL. However for materials, which have a UEL of 100% because they can decompose in the absence of an oxidant, there is no safe region for distillation of the pure material. Use of a diluent gas increases the capital and operating cost of the purification material because it must be bigger and process more material as well as recycle the diluent. Another variation has been to absorb the gas on a solid sorbent as in U.S. Pat. No. 5,518,528. This reduces the pressure in the headspace of the enclosure and stores the majority of the gas as an adsorbed species, which is attached to the sorbent by physical or chemical forces. To desorb the gas from the apparatus it is necessary to reduce pressure, add heat or a combination of the two. As noted in the above patent the sorbent can promote decomposition of the stored gas and special precautions are required to avoid this problem. Adsorption-desorption systems are more complicated for the end user in comparison to a gas cylinder thus they tend to be considerably more expensive to buy and operate. This technique is also not applicable to such standard separation techniques as distillation or membrane separation.
The suppression type of technology relies on the detection of incipient fire or explosion and then rapidly suppressing the event.
U.S. Pat. No. 5,069,291 describes a system of detecting the incipient pressure rise of an explosion and suppressing that explosion by spraying hot pressurized water into the enclosure. The patent claims that the explosion must be detected and suppressed within 10–200 milliseconds and discusses competing technology using chemicals such as Halon 1011 (chlorobromoethane) and MAP (monoammonium phosphate). The use of Halon is being restricted because of concerns about ozone depletion and the use of any suppressant will contaminate the enclosure.
Suppression type systems rely on rapid detection and on the careful design of the flow of the suppressant to the site of the fire or incipient explosion. Suppression of explosions must be much faster than fires and so the detection and activation of the suppressant discharge is advantageously combined as in U.S. Pat. No. 5,069,291. The prior art for direct suppression of fire or explosions is an active system that requires fast detection and response to inject a quenching chemical into the enclosure that may fail or be too slow to prevent the explosion. It is well known to engineers that systems that are not in regular use may not work when needed because of lack of maintenance or an undetected fault. Thus it is normally required to have some routine testing to ensure the equipment still works, which adds cost and may require decontamination of the enclosure after testing. After each suppression of fire or explosion the enclosure is also contaminated with the suppressant materials and some initial products of the reaction. With the increased emphasis on safety the avoidance of flammability approach has become the most common solution because it is essentially a passive system and will prevent fires and explosions within the conditions it is designed for although if the temperature and or the pressure rise above design conditions the gas may become flammable. The major problems are that the diluent gas must be provided and in the case of transportable containers must be also shipped which adds additional cost for the larger container. The larger container also occupies more space during production, distribution and use. Use of lower pressure containers also increases the size of the container and restricting the size of the container increases the cost because it is not possible to obtain economies of scale. A further problem is that the mixture of gases is by definition no longer pure, which is only a minor problem for flammable liquids, since the vapor and diluent mix can be discarded but is a big problem for flammable gases
The lack of purity is particularly a problem during production and purification of materials which have a 100% UEL because there is no safe region for distillation of the pure material in contrast to the ease with which standard purification techniques of distillation and membrane separation can be carried out safely for materials which have a UEL less than 100% simply by operating above the UEL. The option of using a diluent increases the capital and operating cost of the purification material because the separation equipment must be bigger and process more material. Using lower pressure and temperature decreases the risks but also increases the costs as the equipment must be larger and refrigeration equipment increases in cost as the temperature decreases.
Many of the gases with 100% UEL are unstable hydride gases such as hydrazine, acetylene, silane and germane. A hydride gas is a gas containing hydrogen. Unstable gaseous hydrides are gaseous compounds containing hydrogen that have a positive heat of formation. Examples of such gases are found in the compounds of hydrogen with elements from the 4th, 5th and 6th column of the periodic table. The term heat of formation refers to the heat required to form a given compound from the elements of which it is comprised. A negative heat of formation means that heat is given out when the two elements are combined, as an example the formation of water from hydrogen and oxygen provides a large release of heat and conversely the production of the elements from the compound requires the addition of energy. A positive heat of formation means that heat must be added to form the compound from its elements, as an example Silane, SiH4, requires the addition of heat for its formation and generates heat when it decomposes to form the elements. Such gases are very useful in the semiconductor industry for the deposition of pure metals, particularly semiconductors such as silicon and germanium, since they can be produced in a highly purified form by techniques such as distillation. A major disadvantage of such gases is that the decomposition can occur during production of the gases particularly when the gases are condensed under pressure to form liquids suitable for distillation. The decomposition causes a rise in temperature and pressure from the heat release and the production of hydrogen gas. The temperature rise can weaken the construction materials of the container of the gas, which in combination with the rapid pressure rise can result in rupture of the container. The temperature and pressure rise are proportional to the heat of formation. One method for assessing the risk of such decompositions is to calculate the adiabatic temperature and pressure that would occur during decomposition in the absence of any heat loss from the gas and design the container so that these condition can be withstood., See Arno. This latter assumption is fairly valid for the extremely rapid explosive reactions, which are the main concern since heat exchange is much slower than the reaction. These calculations use the known heat of formation to calculate the adiabatic heat release, and then use the known heat capacities of the reaction products produced to calculate the peak temperature produced when all the released heat is used to heat the produced elements. This temperature can be used to calculate the pressure rise due to temperature increase. There is also an increase in the number of gas moles, which contributes further to the pressure rise. The multiplication of these two effects gives the total pressure ratio, which is defined as the final pressure over the initial pressure. The table below gives the calculations based on a starting temperature of 25° C. for typical hydrides.